[GZG] Threshold question using Beta fighter rules

Am playing a series of fleet exercises (i.e.better familiarizing myself
with 
the beta fighter rules) and have a question on Threshold damage
modifier.
The situation:a group of 4 surviving heavy fighters is attacking a NAC 
Troutbridge-class frigate (hull 2/1/1/1) that has already lost first row
of 
hull boxes (3 remaining);one fighter out of the group rolls a beam die
of 
"6" for 2 points damage on its attack run,thus causing Threshold check
to 
the frigate for losing second and third rows of hull boxes.
FB1,p.5 states "If a ship suffers enough damage in a single attack to
push 
it past one Threshold check,make only one check (for last row destroyed)
but 
add 1 to each die roll for extra Threshold point passed in that attack."
So do I check Threshold at 4-6,or is it now 3-6 per system?
I am interpreting it as 3-6 per system due to going past 2 rows damage
at 
once,but want to make sure I am reading it correctly.Never have been too

clear on this point.......

BTW,if anyone is interested in how I am conducting the exercise I can
post 
it here in scenario format.

Thanks,
Mark Drake 
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